Device for detecting radiation including an encapsulating structure having an improved mechanical strength

ABSTRACT

A device for detecting electromagnetic radiation is provided, including a substrate; at least one thermal detector placed on the substrate; and an encapsulating structure encapsulating the detector, including a thin encapsulating layer of a material that is transparent to said radiation, extending around and above the detector so as to define with the substrate a cavity in which the detector is located; wherein the thin encapsulating layer comprises a peripheral wall that encircles the detector, and that has a cross section, in a plane parallel to the plane of the substrate, of square or rectangular shape, corners of which are rounded.

TECHNICAL FIELD

The field of the invention is that of devices for detectingelectromagnetic radiation, and in particular infrared or terahertzradiation, including at least one thermal detector, and preferably amatrix of thermal detectors, and an encapsulating structure that formsat least one hermetic cavity in each of which at least one detector ishoused. The invention is especially applicable to the field of infraredimaging and thermography.

STATE OF THE PRIOR ART

A device for detecting electromagnetic radiation, for example infraredor terahertz radiation, conventionally comprises a matrix of what arereferred to as elementary thermal detectors, each detector including asection able to absorb the radiation to be detected. With the aim ofensuring the thermal insulation of the thermal detectors, each sectionconventionally takes the form of a membrane suspended above thesubstrate and thermally isolated therefrom by thermally insulatingholding elements. These holding elements also provide an electricalfunction as they are used to electrically connect the thermal detectorsto a read circuit generally placed in the substrate.

To ensure optimal detector operation, a low pressure level is required.For this reason, the detectors are generally confined, or encapsulated,whether alone or in groups of two or more, in hermetic cavities that areunder a vacuum or a low pressure.

FIG. 1 illustrates an exemplary detecting device 1 suitable fordetecting infrared radiation, and more precisely one pixel of thedetecting device, here formed by a bolometric detector 2 resting on asubstrate 3 and placed alone in a hermetic cavity 4, such as describedin the publication by Dumont et al., Current progress on pixel levelpackaging for uncooled IRFPA, Proc. SPIE 8353, Infrared Technology andApplications XXXVII, 83531I 2012.

In this example, the detecting device 1 includes an encapsulatingstructure 5, also called a capsule, that defines the cavity 4 in whichthe bolometric detector 2 is located. The encapsulating structure 5includes a thin encapsulating layer 6 that defines with the substrate 3the cavity 4, and a thin sealing layer 7 that covers the thinencapsulating layer 6 and ensures the hermeticity of the cavity 4. Theencapsulating 6 and sealing 7 layers are transparent to theelectromagnetic radiation to be detected.

The detecting device 1 is produced using techniques for depositing thinlayers and especially sacrificial layers. During the production process,the sacrificial layers are stripped and removed from the cavity throughone or more exhaust vents 8 provided in the thin encapsulating layer 6.The sealing layer 7 is used, after the sacrificial layers have beenremoved and the cavity 4 placed under vacuum, to block the exhaust vents8.

With the aim of maximising the fill factor, i.e. the ratio of the areaof the absorbing membrane to the total area of the pixel, in the planeof the substrate, it is desired to decrease as much as possible thespacing between the detectors and the border of the capsules, and thespacing between the capsules in the case where the device provides for aplurality of capsules. To do this, the optimal shape of the capsules, ina plane parallel to the plane of the substrate, is a square orrectangular shape the corners of which are right angles. However, themechanical strength of the capsules remains to be improved.Specifically, a failure in the mechanical strength of a capsule leads tobreakage of the hermeticity of the cavity and therefore possibly to thefunctional loss of the detector, or even of the matrix of detectors.

SUMMARY OF THE INVENTION

The aim of the invention is to at least partially remedy the drawbacksof the prior art, and more particularly to provide a device fordetecting electromagnetic, for example infrared or terahertz, radiationincluding at least one thermal detector placed in a hermetic cavityformed by an encapsulating structure, the mechanical strength of whichis reinforced.

For this purpose, the invention provides a device for detectingelectromagnetic radiation, comprising: a substrate; at least one thermaldetector placed on the substrate; and an encapsulating structureencapsulating the thermal detector, including a thin encapsulating layerextending around and above the thermal detector so as to define with thesubstrate a cavity in which the thermal detector is located, said thinencapsulating layer being produced from a material that is transparentto said electromagnetic radiation.

According to the invention, the thin encapsulating layer comprises aperipheral wall that encircles the thermal detector, and that has across section, in a plane parallel to the plane of the substrate, ofsquare or rectangular shape the corners of which are rounded.

The peripheral wall may include a rounded section and two rectilinearsections extending along axes that are substantially orthogonal to eachother, said rectilinear sections being connected to each other by therounded section, the rounded section having a radius of curvature largerthan or equal to two times the thickness of one of the rectilinearsections.

The radius of curvature of the rounded section may be measured from theinternal surface, i.e. that oriented towards the cavity, of the roundedsection.

The thin encapsulating layer may include at least one through-orificethat is what is referred to as an exhaust vent, said encapsulatingstructure furthermore including a sealing layer covering the thinencapsulating layer so as to make the cavity hermetic, i.e. the sealinglayer hermetically plugs the exhaust vent.

The device may include a matrix of detectors placed in one and the samecavity, the thin encapsulating layer furthermore comprising at least onesection, which is what is referred to as an internal bearing section,located between two adjacent thermal detectors, and which bears directlyagainst the substrate.

The internal bearing section may have a profile, in a plane parallel tothe plane of the substrate of oblong shape, preferably with roundedlongitudinal ends.

The internal bearing section may include a sidewall and a bottomportion, said sidewall extending substantially vertically over theentire height of the cavity and the bottom portion making contact withthe substrate.

The thin encapsulating layer may include at least one through-orificethat is what is referred to as an exhaust vent, having a transverseprofile, in a plane orthogonal to the plane of the substrate, the widthof which increases with distance from the substrate. The encapsulatingstructure may furthermore include a sealing layer covering the thinencapsulating layer so as to make the cavity hermetic, the sealing layerincluding a border that extends in the direction of the thickness of thesealing layer, from the border of the exhaust vent, with a non-zeroangle α relative to an axis substantially orthogonal to the plane of thesubstrate, and in which the transverse profile of the exhaust vent makesan angle β to the same orthogonal axis larger than the angle α.

The detecting device may include a matrix of thermal detectors, eachdetector including a membrane suitable for absorbing the radiation to bedetected, which membrane is suspended above the substrate and thermallyinsulated therefrom by anchoring pins and thermally insulating arms, thethin encapsulating layer including a plurality of through-orifices thatare what are referred to as exhaust vents placed so that at least someof said thermal detectors each have a single exhaust vent located facingthe corresponding absorbing membrane, preferably plumb with the centreof said membrane. In other words, the single vent is located plumb withthe absorbing membrane, i.e. perpendicular to the absorbing membrane.The single vent is therefore not located facing anchoring pins orthermally insulating arms.

Each absorbing membrane may include a through-orifice facing thecorresponding exhaust vent, of larger or equal size to that of saidvent. In other words, said absorbing membrane, facing which is locatedan exhaust vent, may include a through-orifice located plumb with saidexhaust vent and of larger or equal size to that of said vent.

The suspended membrane may include a stack of a bolometric layer, adielectric layer that is structured so as to form two separate sections,and an electrically conductive layer that is structured so as to formthree electrodes, two of said electrodes, which are intended to beraised to the same electrical potential, flanking the third electrode,which is what is referred to as the central electrode and which isintended to be raised to a different electrical potential, eachelectrode making contact with the bolometric layer, the centralelectrode being electrically insulated from the other electrodes by thedielectric layer, the through-orifice passing through the centralelectrode and the bolometric layer in a zone located between thesections of the dielectric layer.

The encapsulating structure may furthermore include a sealing layercovering the thin encapsulating layer so as to make the cavity hermetic,and in which the substrate comprises a tie layer placed facing thethrough-orifice of the corresponding membrane and suitable for ensuringthe adhesion of the material of the sealing layer.

The tie layer may extend under the whole of the corresponding membraneand may be furthermore made of a material suitable for reflecting theelectromagnetic radiation to be detected.

The invention also relates to a process for producing a detecting devicehaving any one of the above features, in which the thin encapsulatinglayer is placed on a sacrificial layer, the latter then being removedand extracted through at least one exhaust vent in the thinencapsulating layer.

BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects, aims, advantages and features of the invention willbecome more clearly apparent on reading the following detaileddescription of preferred embodiments thereof, this description beinggiven by way of nonlimiting example and with reference to the appendeddrawings in which:

FIG. 1, described above with reference to an example of the prior art,is a schematic cross-sectional view of a detecting device according toone embodiment;

FIG. 2 is a schematic representation of a top view of a detecting deviceaccording to one embodiment, comprising a matrix of detectors in whicheach detector is housed in a separate cavity, the cavities having ashape with rounded corners;

FIGS. 3 and 4 are partial schematic representations of top views of theperipheral wall of the thin encapsulating layer according to oneembodiment, in which the wall includes, at the corners of the cavity, arounded section;

FIG. 5 is a schematic representation of a top view of a detecting deviceaccording to one embodiment, in which a matrix of detectors is housed inone and the same cavity;

FIG. 6 is a cross-sectional schematic view in the plane A-A of thedetecting device shown in FIG. 5;

FIG. 7 is a cross-sectional schematic view in the plane B-B of thedetecting device shown in FIG. 5;

FIGS. 8 to 10 are cross-sectional schematic views of a detecting deviceaccording to one embodiment, at various stages of its productionprocess;

FIG. 11 is a schematic representation of a top view of an exhaust ventaccording to another embodiment, in which the vent has a oblong-shapedprofile with rounded ends;

FIG. 12 is a partial cross-sectional view of a portion of a detectingdevice according to one embodiment;

FIG. 13 is a cross-sectional schematic view of a detecting deviceaccording to one embodiment, in which a single exhaust vent per detectoris placed facing the suspended membrane and in which the membraneincludes a through-orifice located plumb with the exhaust vent; and

FIGS. 14 and 15 are schematic views of a detecting device according toanother embodiment, in which the suspended membrane includes anintermediate dielectric layer.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

In the figures and in the rest of the description, references that arethe same represent identical or similar elements.

FIG. 1, which was incompletely described above with reference to anexample of the prior art, here illustrates an exemplary device fordetecting electromagnetic radiation according to one embodiment.

In this example, the device 1 for detecting electromagnetic radiation issuitable for detecting infrared or terahertz radiation. It includes amatrix of thermal detectors 2 that are what are referred to aselementary detectors. FIG. 1 is a partial view of the detecting deviceand shows only a single detector placed in a cavity.

It comprises a substrate 3, for example made of silicon, comprising aread circuit (not shown) for example produced in CMOS technology,allowing the biases required to operate the detectors and read theinformation issued therefrom to be applied.

The thermal detector 2 comprises a section suitable for absorbing theradiation to be detected. This absorbing section is generally thermallyinsulated from the substrate and may be placed on a membrane 9, which iswhat is referred to as an absorbing membrane, suspended above thesubstrate 3 by thermally insulating holding elements 11 such asanchoring pins 11 a associated with thermally insulating arms 11 b. Themembrane 9 is spaced apart from the substrate 3 by a distance typicallycomprised between 1 μm and 5 μm, and preferably 2 μm when the detectoris designed to detect infrared radiation the wavelength of which iscomprised between 8 μm and 14 μm.

In the rest of the description, the thermal detectors 2 are bolometersthe absorbing membrane 9 of which includes a thermistor material theelectrical conductivity of which varies as a function of the temperatureof the membrane. However, this example is given by way of illustrationand is nonlimiting. Any other type of thermal detector may be used, forexample ferroelectric or pyroelectric detectors or even thermopiles.

In one example detailed below with reference to FIG. 5, the detectors 2,which are placed in one and the same cavity, may be arranged closetogether, especially by connecting the thermally insulating arms 11 b ofvarious thermal detectors to a given anchoring pin 11 a, the readarchitecture of the thermal detectors then being adapted, as documentsEP1106980 and EP1359400 describe. An improvement in the sensitivity ofthe detectors 2 results due to the increased length of the insulatingarms 11 b and the increase in the fill factor achieved by decreasing thearea of each pixel not dedicated to the absorption of electromagneticradiation. The detecting device is thus particularly appropriate forsmall matrix pitches, for example between 25 μm and 17 μm, or even 12μm.

The detecting device 1 includes an encapsulating structure 5, orcapsule, that defines, with the substrate 3, a hermetic cavity 4 insideof which the thermal detector 2 is placed. The encapsulating structure 5is formed from a thin encapsulating layer 6 that is deposited so that ithas a peripheral wall 6 a that encircles the detector 2 and a top wall 6b that extends over the detector 2. The top wall 6 b is substantiallyplanar and extends over the suspended membrane 9 at a distance therefromfor example comprised between 0.5 μm and 5 μm, preferably 1.5 μm.

The encapsulating layer 6 is a thin layer in that it has a thicknesssmaller than a few tens of microns, for example than 10 μm, and in thatit is produced by layer deposition techniques conventionally used inmicroelectronics and especially conformal deposition techniques such asCVD (Chemical Vapour Deposition) or iPVD (Ionised Physical VapourDeposition).

Moreover, the encapsulating layer 6 is produced from a material that istransparent to the electromagnetic radiation to be detected. Bytransparent material, what is meant is a material the factor oftransmission of the radiation to be detected of which is higher than20%, preferably than 50%, preferably than 80% and more preferably than90%. In the case of infrared radiation, for example in the wavelengthrange comprised between 8 μm and 14 μm, an exemplary transparentmaterial is silicon, germanium or silicon-germanium SiGe, for exampleamorphous silicon, the transmission factor of which is here higher than95%.

As described below, the encapsulating layer 6 is preferably producedusing a freed layer technique, in so far as it is deposited on asacrificial layer that covers the detector, the latter then beingremoved and extracted through at least one through-orifice in theencapsulating layer, this orifice being what is referred to as anexhaust vent. This production technique in particular differs from whatare referred to as transferred layer techniques, in which the thinencapsulating layer is produced on a structured substrate separate fromthe substrate 3, then added to and assembled with the substrate 3.

FIG. 2 is a schematic representation of a top view of a matrix ofdetectors 2 in which each detector is placed in a separate hermeticcavity from its neighbours. The thin encapsulating layer 6 is depositedso as to extend around and over each detector. Thus, a plurality ofcapsules 5 is formed. Each capsule here has a cross section, in a planeparallel to the plane of the substrate, of square or rectangular shapewith rounded corners.

The inventors have observed that the production of rounded sections atthe corners of the capsule improves the adherence of the latter to thesubstrate. Specifically, it has been observed that the adherence of thecapsule is not uniform along the peripheral wall and that the corners ofthe capsule have a reinforced adherence when rounded sections areproduced.

As FIGS. 3 and 4 show, the peripheral wall 6 a of the thin encapsulatinglayer 6 is formed, at each corner of the capsule, from two sections 6a-1, 6 a-2 extending substantially rectilinearly, each along an axis X1,X2 that is substantially orthogonal to the other. The rectilinearsections 6 a-1 and 6 a-2 do not join at a right angle but are connectedto each other by a rounded section 6 a-3.

By rounded section, what is meant is a section having at least onecurved and for example circular or elliptical segment, or at least oneright segment, and preferably a plurality of right segments, extendingalong an axis that is not colinear with the respective axis of therectilinear sections.

FIG. 3 shows an example of a rounded section 6 a-3 taking the form of acircular arc segment connecting the rectilinear sections 6 a-1 and 6a-2. The radius of this circular arc, measured from the external surfaceof the rounded section 6 a-3, i.e. the surface oriented towards theexterior of the cavity (escribed circle), may be larger than or equal totwo times the width L of the peripheral wall. Preferably, the dimensionsof the rounded section are such that the radius of an inscribed circle,i.e. a circle tangent to the internal surface oriented towards thecavity, of the rounded section is larger than or equal to two times thewidth L.

The width L is defined as the average width of a substantiallyrectilinear section 6 a-1, 6 a-2 of the peripheral wall 6 a. The roundedsection 6 a-3 preferably has a width substantially equal to that of therectilinear sections.

FIG. 4 shows another example of a rounded section, as a variant to thatin FIG. 3. In this example, the rounded section 6 a-3 is formed by asuccession of two right segments that are inclined one relative to theother. It is possible to define an escribed circle, tangent to theexternal surface of each segment. The orientation of the segments may besuch that the radius of the escribed circle is larger than or equal totwo times the width L of the peripheral wall. Preferably, theorientation of the segments is such that the radius of an inscribedcircle, i.e. a circle tangent to the internal surface of the segments,is larger than or equal to two times the width L.

By way of example, the width L of the peripheral wall of the thinencapsulating layer may be comprised between about 200 nm and 2 μm. Theradius of the inscribed or escribed circle is larger than or equal to avalue comprised between 400 nm and 4 μm depending on the width L, forexample 2 μm in the case of a width L equal to 800 nm.

According to one embodiment, shown in FIGS. 5 to 7, the detecting deviceincludes a detector matrix 2 housed in one and the same hermetic cavitywith rounded corners. The encapsulating structure 5 includes at leastone internal bearing section 12 located between two adjacent detectors2, and preferably a plurality of internal bearing sections. Certaininternal bearing sections may furthermore be placed on the periphery ofthe matrix of detectors 2, bordering the cavity 4. The internal bearingsections 12 are formed by the thin encapsulating layer 6, which thusincludes continuously the peripheral wall 6 a, the top wall 6 b and theinternal bearing sections 12.

The internal bearing sections 12 rest (or bear) directly on (or against)the substrate 3. In other words, they make direct contact with thesubstrate. These internal bearing sections 12 thus allow the mechanicalstrength of the capsule 5 to be reinforced. The adherence of the capsule5 to the substrate 3 is thus ensured on the one hand by a bottom portionof the peripheral wall 6 a of the thin encapsulating layer 6, whichrests on the substrate on the periphery of the cavity, and on the otherhand by the one or more internal bearing sections 12 placed in thecavity.

This multiplicity of contact areas, distributed bordering the cavity andinside thereof, makes it possible to increase the mechanical strength ofthe capsule. In addition, in so far as the capsule also has roundedcorners, the overall adherence of the capsule to the substrate isparticularly reinforced, by virtue of the synergistic effect between themultiplicity of bearing areas deposited in the cavity and the localreinforcement of the adherence at the corners of the cavity.

By resting directly on or bearing directly against the substrate, whatis meant is that the internal bearing sections 12 make direct contactwith the substrate 3, independently of whether this is with the materialmaking up the substrate or with a thin layer, for example a passivatinglayer or a tie layer, deposited on the surface of the substrate, andindependently of whether these thin layers extend continuously or not.The internal bearing sections therefore do not rest on the substrate viathree-dimensional elements such as the elements holding the suspendedmembranes.

Specifically, the inventors have observed that, when the bearingsections of the thin encapsulating layer rest, not on the substrate, buton the elements holding the suspended membranes and more precisely onthe anchoring pins, problems arise with adhesion of the capsule to thesubstrate, which may lead to debonding or even destruction of thecapsule. Specifically, it appears that the anchoring pins provide acontact area and planarity that are insufficient to ensure a goodadhesion of the bearing sections of the thin encapsulating layer. Thedetecting device according to the invention thus decreases the risk ofthe capsule debonding, this risk being related to mechanical stresses inthe thin layers of the capsule, whether it be a question of stressesthat are intrinsic to said thin layers or extrinsic stresses resultingfrom differential thermal expansion of the capsule with respect to thesubstrate.

Thus, the encapsulating structure 5 defines a hermetic cavity 4 thathouses the matrix of thermal detectors 2, this cavity 4 taking the formof a network of intercommunicating sub-cavities, or cells, that eachhouse a thermal detector subassembly. The cells are separated from eachother by the internal bearing sections. As explained above, this networkof cells is delimited by one and the same thin encapsulating layer 6,which extends continuously so as to form the peripheral wall 6 a and topwall 6 b of the cavity 4 and the internal bearing sections 12.

Thus, the device 1 for detecting radiation includes a hermetic cavity 4that houses a plurality of thermal detectors 2, the mechanical strengthof the cavity being reinforced by the presence of the one or moreinternal bearing sections 12 that rest directly on the substrate 3.Housing a plurality of thermal detectors 2 in the cavity allows the fillfactor to be increased, for example by decreasing the matrix pitch or byincreasing the size of the absorbing membranes 9, or even by mutualisingthe anchoring pins 11 a. Moreover, parasitic electrical coupling betweendetectors 2 is avoided in so far as the internal bearing sections 12 donot make contact with the anchoring pins. This device furthermore allowsthe length of the thermally insulating arms 11 b to be increased inorder to improve the thermal insulation of the absorbing membranes 9.

FIG. 6 is a cross-sectional view in the plane A-A of the detectingdevice 1 shown in FIG. 5. It shows in greater detail the thinencapsulating layer 6 extending continuously around and above the matrixof detectors 2 so as to form the cavity 4. The peripheral wall 6 a formsthe border of the cavity and the top wall 6 b extends above thedetectors 2. The peripheral wall 6 a has a peripheral bottom portion 6 cthat bears (or rests) directly against (or on) the substrate, so as toensure adhesion of the capsule to the substrate.

FIG. 7 is a cross-sectional view in the plane B-B of the detectingdevice 1 shown in FIG. 3. In this figure, the internal bearing sections12 each comprise a peripheral sidewall 12 a and a bottom portion 12 b,and bear directly against the substrate 3 via the bottom wall 12 b. Inother words, each internal bearing section 12 makes contact directlywith the substrate 3, whether this be with the constituent material ofthe substrate 3 or, as mentioned above, with a thin layer deposited onthe surface of the substrate.

As shown in FIG. 5, the internal bearing sections 12 may have a profile,in the plane of the substrate, of oblong, i.e. elongate, shape. They mayeach be placed between two adjacent suspended membranes and twoneighbouring anchoring pins, so as to optimise the fill factor. The endsof the oblong profile of the internal bearing sections 12 may berounded, so as to strengthen the adherence of the latter to thesubstrate 3 by improved distribution of mechanical stresses. The widthof the internal bearing sections may be smaller than 1.5 μm and forexample comprised between 0.5 μm and 0.8 μm, and their length may beadjusted depending on the space available between the detectors andespecially the anchoring pins.

In the example in FIG. 5, the thermally insulating arms 11 b mainlyextend along a first axis here perpendicular to the cross-sectionalplane B-B, and the internal bearing sections 12 of the capsule 5 extendalong a second axis orthogonal to the first axis, here colinear with thecross-sectional plane A-A, between two adjacent membranes 9 and twoneighbouring anchoring pins 11 a. The width and length of the internalbearing sections may be optimised by taking advantage of the area leftfree in this zone by the absence of thermally insulating arms. The areaof the internal bearing sections making contact with the substrate maythus be large, thereby improving the adherence and mechanical strengthof the capsule.

An exemplary production process is now detailed, with reference to FIGS.8 to 10, which are cross-sectional views, along the axis C-C, of thedetecting device shown in FIG. 5.

The detecting device 1 comprises a substrate 3 in which a circuit forreading and controlling the thermal detectors 2 is provided. Thesubstrate 3 may comprise a passivating layer 13, for example made ofsilicon oxide SiO or silicon nitride SiN. According to one embodimentthat is detailed below, the substrate 3 may also comprise an optionallycontinuous tie layer 14 deposited on the passivating layer 13. The tielayer 14 may be made from titanium or chromium, and have a thickness forexample comprised between about 100 nm and 300 nm.

As is known per se, a first sacrificial layer 15 is deposited and theanchoring pins 11 a, the thermally insulating arms 11 b and theabsorbing membranes 9 are produced in and on this sacrificial layer 15.The sacrificial layer may be made from polyimide or even an inorganicmaterial such as silicon oxide, polysilicon or amorphous silicon.

As illustrated in FIG. 9, a second sacrificial layer 16 is thendeposited on the first sacrificial layer 15, anchoring elements 11 a,thermally insulating elements 11 b and absorbing membranes 9. This layeris preferably made of the same material as that of the first sacrificiallayer 15 and has a thickness for example comprised between 0.5 μm and 5μm.

Steps of photolithography and etching, for example RIE etching, arecarried out so as to form, preferably during a sequence of common steps,trenches 17, 18 right through the thickness of the sacrificial layers,i.e. as far as the substrate 3 and, more precisely, here as far as thetie layer 14. A first trench 17 is produced so as to extend continuouslyaround the matrix of detectors 2 and is intended for the subsequentproduction of the peripheral wall of the encapsulating structure. Thetrench 17 is produced so that the final shape of the capsule, in a planeparallel to the plane of the substrate, has a square or rectangularshape with rounded corners.

At least one and preferably a plurality of second trenches 18, areproduced between two adjacent detectors 2 with the aim of formingsubsequently the internal bearing section(s). The first and secondtrenches 17, 18 have a substantially identical depth, so that theperipheral wall of the encapsulating structure and the sidewalls of thebearing sections have in fine a substantially identical height. Theprocess is thus simplified, especially as regards the control of theetch depth.

In the case where the sacrificial layers 15, 16 are made of polyimide,the process for producing the trenches may involve depositing aninorganic protective layer (not shown), for example made of SiN or SiO,or even of amorphous silicon, on the surface of the second sacrificiallayer 16. A photolithography step then allows apertures to be defined ina resist layer in the locations where the etching of the trenches is tobe carried out. The etching of the trenches is then carried out in 2steps, a first step in which the protective layer is etched, for exampleby RIE etching, plumb with the apertures in the resist, and a secondstep in which the first and second sacrificial layers are etched, forexample by RIE etching, as far as the substrate, plumb with theapertures obtained in the protective layer in the first etching step. Atthis stage, the protective layer may be removed.

This sequence of steps is justified by constraints on the chemicalcompatibility of the layers present and by geometric constraints (aspectratio of the trenches). Specifically, the resist layer would disappearin the second step of etching of the polyimide as these layers are allof organic nature, and therefore similarly sensitive to the etchingchemistry implemented in the second step. The aperture in the protectivelayer is then used as a relay to continue to limit the etching to thezones in which it is desired to produce the trenches. The process of thesecond etching step is moreover adapted to guarantee a high etchinganisotropy, thereby allowing high aspect ratios and vertical sidewalls(orthogonal to the plane of the substrate) to be obtained withoutundercutting. It is furthermore adapted to guarantee a high selectivityon the one hand over the protective layer (made of SiN or SiO) and onthe other hand over the surface of the substrate, generally covered withan insulating passivating layer made of SiO or SiN. This highselectivity is advantageous because it allows the thickness of theprotective layer to be decreased (typically to 30 nm), this being of anature to facilitate its subsequent removal.

The trenches 17, 18, and especially the second trenches 18 intended forproduction of the internal bearing sections, have a high aspect ratio.By way of example, trenches of width smaller than or equal to 1.5 μm,for example comprised between 0.5 μm and 0.8 μm, may be produced in apolyimide layer of thickness comprised between 2 μm and 6 μm, 4 μm forexample. The length of the second trenches 18 may be adapted dependingon constraints on the compact integration and robustness of the capsule,and may be about a few microns to a few millimeters. These trenchdimensions make it possible to produce a matrix of thermal detectorshaving a particularly small matrix pitch, for example 17 μm or even 12μm.

The tie layer 14 is preferably made from a material over which theetching of the trenches is selective, so as to avoid any etching of thesubstrate. The material may be titanium or chromium and the tie layermay have a thickness of about 100 nm to 300 nm.

As shown in FIG. 10, a thin encapsulating layer 6, which is transparentto the radiation to be detected, is then deposited using a conformaldeposition technique suitable for obtaining a good coverage of thevertical flanks of the trenches 17, 18, with a substantially constantlayer thickness. It may for example be a question of an amorphoussilicon layer, a germanium or silicon-germanium layer, produced by CVDor by iPVD, of a thickness typically comprised between about 200 nm and2000 nm when it is measured on a flat surface. The deposition of thethin encapsulating layer 6 on a surface structured with trenchesincluding at least one continuous peripheral trench 17 (closed perimeterof square or rectangular shape, with rounded corners) leads to theformation of the capsule 5, produced with the material of the thinencapsulating layer and forming, making contact with the substrate 3, acavity 4 in which the matrix of detectors is housed. The coverage of theflanks of the internal trenches 18 by the thin encapsulating layer 6allows the shape of the internal trenches to be reproduced in order toform internal bearing sections 12, preferably of oblong shape withrounded edges. It will be noted that these internal bearing sections 12may be solid or hollow (made up of two spaced-apart walls) depending onwhether the width of the internal trenches 18 is small or large relativeto the thickness of the thin encapsulating layer 6, respectively.

Through-orifices, forming exhaust vents 8 intended to allow the removalof the sacrificial layers 15, 16 from the cavity, are then produced byphotolithography and etching in the thin encapsulating layer 6. Eachvent 8 may be square, rectangular, circular, or even oblong in shape.

Next, the sacrificial layers 15, 16 are removed by preferably gas-phaseor vapour-phase (depending on the nature of the sacrificial layers)chemical attack (gas-phase attack is used in the polyimide casedescribed here), so as to form the cavity 4 housing the matrix ofdetectors 2, and the internal bearing sections 12.

A sealing layer (not shown in FIG. 10) is then deposited on the thinencapsulating layer 6 with a sufficient thickness to ensure the exhaustvents 8 are sealed or blocked. The sealing layer is transparent to theelectromagnetic radiation to be detected and may have an antireflectionfunction in order to optimise the transmission of radiation through theencapsulating structure. In this respect, it may be formed fromsublayers of germanium and zinc sulphide in the case where the radiationto be detected lies in the wavelength range extending from 8 μm to 12μm, for example a first sublayer of germanium of about 1.7 μm thicknessthen a second sub-layer of zinc sulphide of about 1.2 μm thickness. Thesealing layer is preferably deposited by a vacuum thin-film depositiontechnique such as electron-beam vacuum evaporation (EBPVD) or such asion beam or cathode sputtering. Thus a hermetic cavity 4 under vacuum orlow pressure is obtained in which the matrix of thermal detectors 2 ishoused.

According to another embodiment (not shown), which differs from theexample in FIG. 5 in which only a single internal bearing section 12 isproduced between two adjacent detectors 2, in that a plurality ofinternal bearing sections, here two internal bearing sections of oblongprofile, extend longitudinally along the same axis, and are locatedbetween two adjacent detectors 2. As in the example in FIG. 5, thelongitudinal axis of the internal bearing sections 12 may besubstantially perpendicular to the axis along which the insulating arms11 b mainly extend. Increasing the number of internal bearing sections12 allows the adherence of the capsule 5 to the substrate 3 to bereinforced and thus the mechanical strength of said capsule to bereinforced.

According to another embodiment (not shown) each detector 2 may beconnected to four anchoring pins 11 a certain of which are common to twodirectly neighbouring detectors located in the same column (or in thesame row). This architecture both allows the mechanical strength of thesuspended membranes 9 to be improved and permits the matrix of detectorsto be read sequentially row by row (or column by column, respectively)as it is conventional to do using reading means located, at the end ofthe column (at the end of the row, respectively), in a read circuitproduced in the substrate of the device. The sensitivity of thedetectors is improved with this shared anchoring pin architecture as thelength of the thermally insulating arms 11 b may be increased, and thefill factor is improved by the mutualisation of the anchoring points 11a, which do not contribute to the capture of the infrared signal.

In this example, the internal bearing sections 12 of the capsule 5 arepreferably positioned at the repetition pitch of the detectors, in thetwo dimensions of the matrix of detectors. The shape of the bearingsections 12 is essentially linear and those that are colinear with theinsulating arms 11 b are advantageously arranged between the arms of thedetectors 2 of a given row. Positioning the bearing sections along twoaxes is of a nature to reinforce the adherence of the capsule to thesubstrate.

Advantageously, internal bearing sections 12 may also be producedbetween the edge detectors and the peripheral wall 6 a of the capsule 5.These additional bearing sections essentially have the function ofrecreating, for the edge detectors, an environment (especially from anoptical point of view) comparable to that of core detectors. Another wayof decreasing these edge effects would be to provide, on the peripheryof the matrix, rings of dummy detectors that would not contribute to thevideo signal of the matrix-array device. Rings of one to a few,typically two, detectors satisfactorily provide this function.

According to one embodiment shown in FIG. 11, the profile of the exhaustvents 8, in a plane parallel to the plane of the substrate, has anoblong, i.e. elongate, shape. Its small dimension X, measured in thedirection of the width of the vent, is chosen so as to ensure effectivesealing of the vent, and its large dimension Y, measured in thedirection of the length of the vent, may be adjusted to facilitate thetransit of reactive species and reaction products of the etching of thematerial of the sacrificial layers 15, 16 during removal, therebyallowing the time taken to remove the sacrificial layers to beoptimised. In this respect, the width X may typically be comprisedbetween about 150 nm and 600 nm, whereas the large dimension Y may beabout a few microns, 5 μm for example.

Advantageously, the vents 8 have an oblong shape with roundedlongitudinal ends. By way of example, the rounded shape of an end mayhave a radius of curvature equal to half the width X of the vent. Moregenerally, it may correspond to a continuous, circular or elliptical,curved shape, such as in the example in FIG. 11, or to a succession ofright or substantially curved segments. The inventors have shown thatthis vent shape makes it possible to avoid the risk of cracks initiatingin the thin encapsulating layer 6 and propagating into the sealing layer7. Specifically, it is essential to prevent any risk of cracks that areliable to break the hermeticity of the cavity, in so far as a localhermeticity flaw could lead to operational failure of the entire device.

As FIG. 12 shows, the inventors have observed that the sealing layer 7,bordering the vents 8, has a tendency to extend vertically, i.e. in thedirection of the thickness of the layer 7, with a non-zero angle αrelative to the normal, i.e. relative to an axis orthogonal to the planeof the substrate, in particular when a vacuum thin-film depositiontechnique, such as low-pressure sputtering or evaporation, is used. Theaverage width X of the vents may be chosen depending on the thickness eof the deposited sealing layer 7, on the fractional thickness B of thesealing layer actually ensuring the hermeticity, and on the growth angleα, from the following relationship:X=2·e(1−B)·tan(α)

By way of example, when an evaporation technique is used to deposit thesealing layer, the angle α is typically about 15° to 20°. For athickness e of sealing layer of 1800 nm, and if it is desired for 1200nm of layer to ensure the hermeticity (B=⅔), an average vent width X ofabout 320 nm to 410 nm is obtained.

Moreover, it is advantageous for the exhaust vent 8 to have a crosssection, in a plane orthogonal to that of the substrate, that has ashape the aperture of which increases in size with distance from thesubstrate 3. In other words, the vent 8 has a transverse profile that isflared towards the exterior of the cavity. It is narrower level with itsbottom orifice opening onto the cavity and wider level with its toporifice opening outside of the cavity. By way of example, the widthX_(inf) level with the bottom orifice may be about 100 nm to 350 nmwhereas the width X_(sup) level with the top orifice may be about 250 nmto 800 nm. In this example, the thin encapsulating layer 6 has athickness of about 800 nm. It results from this shape of the crosssection of the vent 8 that the quality of the seal sealing the vent isimproved. More precisely, for a given thickness e of sealing layer, theinventors have observed that the fraction B of layer that actuallyprovides the seal is larger in the case where the vent has a right crosssection, thereby improving the quality of the seal.

Such a vent cross section may be obtained by generating a slope in theflanks of the resist before etching of the vent, either bypost-development reflow or by modifying the conditions of exposureand/or development of the resist (exposure dose, focus, temperature andduration of post-exposure anneals) as is known by those skilled in theart. Such a vent cross section may also be obtained during the dryetching of the vent by adding an isotropic component to the etching, forexample by adding oxygen to the chemistry used to etch the vent. In thecase where the thin encapsulating layer 6 is made of silicon, theaddition of fluorine-containing gases, such as SF₆ or CF₄, to the etchchemistry will also contribute to increasing the isotropic component ofthe etching.

The beneficial effect of this particular vent profile especiallymanifests itself when the angle β that the profile of the vent makes tothe normal to the substrate is larger than the angle α defined above. Byway of example, for a thin encapsulating layer thickness of 800 nm andfor a bottom orifice width X_(inf) of 100 nm, the top orifice widthX_(sup) may be larger than 530 nm (β=15°), or even larger than 680 nm(β=20°). In the embodiment in FIG. 12, the vent 8 is placed on theborder of the cavity 4 but it could be located at other cavitylocations.

In this respect, according to one embodiment illustrated in FIG. 13, thethin encapsulating layer 6 includes at least one exhaust vent 8 placedso that at least one thermal detector 2 present in the cavity 4 has asingle exhaust vent 8 located facing its absorbing membrane 9,preferably plumb with the centre of the absorbing membrane 9. In otherwords, the single vent 8 is located plumb with the absorbing membrane 9,i.e. perpendicular to the absorbing membrane 9. The single vent 8 istherefore not located facing anchoring pins 11 a or thermally insulatingarms 11 b.

Thus, production of the vent is simplified by its distance from thezones of high topography that are the trenches, thereby allowing a gooddimensional control of the shape of the vent to be obtained. Inaddition, the inventors have observed that positioning a single ventfacing the absorbing membrane of the thermal detector makes it possibleto avoid, after removal of the sacrificial layers, the presence ofsacrificial layer residues attached to the membrane. The presence ofthese residues has especially been observed when at least two vents perdetector are placed on either side of the membrane. The residues aregenerally located in a zone equidistant from the various vents, thesuspended membrane being located in said zone. They may modify theoptical and/or electrical and/or thermal properties of the membrane (forexample by increasing the mass of the membrane, thereby decreasing theresponse time of the detector), or even modify the residual pressurelevel under the effect of gradual degassing. In addition, the step ofremoving the sacrificial layers is optimised, especially in terms of thetime taken to remove the sacrificial layers, by way of a combined effectof the oblong shape of the vent and the central position thereof withrespect to the detector.

In the case where the cavity 4 houses a single thermal detector 2, thethin encapsulating layer 6 then comprises a single exhaust vent 8located facing the absorbing membrane 9 of the thermal detector. Ingeneral, the detecting device includes a matrix of thermal detectors 2in which each detector is encapsulated in a single cavity. Theencapsulating structure then includes a matrix of cavities all formed bythe same thin encapsulating layer. Level with each cavity, the thinencapsulating layer includes a single exhaust vent placed facing theabsorbing membrane of the detector housed in the cavity.

In the case where the cavity 4 houses a plurality of thermal detectors2, the thin encapsulating layer then includes at least one exhaust ventand preferably a plurality of exhaust vents placed so that at least someof said thermal detectors 2 each has a single exhaust vent 8 locatedfacing the corresponding absorbing membrane 9. Each thermal detector ofthe matrix may have a single vent placed facing the correspondingabsorbing membrane. Alternatively, only some of the thermal detectorsmay each have a single exhaust vent located facing the correspondingmembrane. It is then advantageous, for a row or a column of thermaldetectors, for the exhaust vents to be placed above every Nth unevendetector. This makes it possible to avoid the presence of sacrificiallayer residues on the absorbing membrane of a detector not provided withan exhaust vent. By way of example, in the case where N=3, twoneighbouring detectors not provided with an exhaust vent are placedbetween two detectors each provided with a single exhaust vent. In thisexample, none of the thermal detectors, whether they are or are notprovided with an exhaust vent, will see their absorbing membranedegraded by the presence of sacrificial layer residues. This variantembodiment is particularly advantageous in the case of small matrixpitches, for example when the positional pitch of the detectors is about12 μm or less.

It is then advantageous to provide a through-orifice 19 in the membrane9 of the detector, located plumb with the corresponding vent 8, and thesize of which is equal to or larger than the size of the vent 8, so asto achieve a margin of safety allowing for possible misalignment of thevent and/or orifice of the membrane, which may be about 200 nm to 500nm. Thus, during the deposition of the sealing layer, a section of thesealing material that is liable to fall through the vent will not bedeposited on the membrane but will instead pass through the orifice ofthe membrane and be deposited on the substrate.

It is then advantageous to provide (on the substrate) a tie layer, underthe membrane 9, in line with the through-orifice 19, in order to ensurethe fallen sealing agglomerate, if there is one, remains held in place.Advantageously, this tie layer may be a section of the aforementionedtie layer 14, the material of which is then suitable for furthermoreensuring the attachment of the sealing material. Thus, in the step ofsealing the cavity, in the case where a quantity of sealing layermaterial passes through the vent, said quantity will deposit on andadhere to the tie layer. This especially makes it possible to relaxconstraints on the type of material present on the surface of thesubstrate, and more precisely on the material used to passivate the topside of the substrate.

This tie layer 14 may extend, continuously or discontinuously, overvarious zones of the cavity, and more precisely: under the membrane 9and facing its through-orifice 19, in order to ensure attachment of thesealing material liable to fall through the vent 8; under the entiretyof the membrane 9 in order to provide an optical function enablingreflection of the radiation to be detected; level with the varioustrenches 17, 18 in order to protect the substrate 3 during the etchingstep used to form the trenches and to improve the attachment of the thinencapsulating layer 6 to the substrate; and level with the anchoringpins 11 a in order to improve the attachment of the pins to thesubstrate and improve the electrical conduction between the pins and theread circuit placed in the substrate. The thickness of this tie layer ispreferably constant over its entire extent, and especially in thevarious aforementioned zones. This tie layer may be made of chromium ortitanium, aluminium or titanium nitride or of another suitable materialand optionally takes the form of a stack of sublayers made from theaforementioned materials, and have a thickness of about 100 nm to 400nm.

According to one embodiment shown in FIGS. 14 and 15, the detectors 2the membrane 9 of which includes a through-orifice 19 have a membranearchitecture with an intermediate electrical insulation, such asdescribed in document EP1067372.

FIG. 14 is a top view of an absorbing membrane 9 of a bolometricdetector with this type of architecture. The absorbing membrane 9 isconnected to four anchoring pins 11 a and is suspended by way of twothermally insulating arms 11 b. FIG. 15 is a cross-sectional view in theplane A-A in FIG. 14.

The membrane 9 includes a layer of a bolometric (and thereforeresistive) material 20, for example doped amorphous silicon or vanadiumoxide. It also includes a layer of a dielectric material 20 that isplaced on the bolometric layer 20 and that covers the latter in twoseparate zones 21 a, 21 b.

It also includes a layer of an electrically conductive material 22,which layer is deposited on the dielectric layer 21 and the bolometriclayer 20 and locally etched over the entire width of the membrane as faras the dielectric layer, so as to form three separate conductivesections 22 a, 22 b, 22 c. The conductive layer 22 extends onto theinsulating arms 11 b in order to electrically connect the three sections22 a, 22 b, 22 c to the read circuit. Among the three conductivesections, the two sections 22 a, 22 c that are located at the ends ofthe membrane 9 are electrically connected to two portions of the sameinsulating arm 11 b and thus form two electrodes intended to be raisedto the same electrical potential. These two end sections 22 a, 22 cflank a central portion 22 b connected to another insulating arm thatforms an electrode intended to be raised to another electricalpotential.

The dielectric layer 21 is etched so that each electrode 22 a, 22 b, 22c makes electrical contact with the bolometric material 20 and so thatthe end electrodes 22 a, 22 c are electrically insulated from thecentral electrode 22 b.

In this embodiment, the absorbing membrane 9 includes a through-orifice19, here of oblong profile, placed at the centre of the centralelectrode 22 b. Preferably, the orifice 19 is placed level with wherethe dielectric layer 21 is etched. The orifice 19 thus only passesthrough the central electrode 22 b and the bolometric layer 20.Preferably, the distance, measured in the direction of the width of theorifice 19, between the border of the orifice and the border of thedielectric layer 21, facing the orifice, is larger than or equal to thethickness of the bolometric layer 20 making contact with the centralelectrode 22 b in this zone. Any influence the orifice might have on theelectrical properties of the absorbing membrane is minimised or evensuppressed by positioning the orifice in this way.

The example described with reference to FIGS. 14 and 15 shows abolometric layer 20 in the bottom portion of the membrane 9, on whichthe dielectric layer 21 and the electrodes 22 a, 22 b, 22 c rest.However, an inverted arrangement of the layers is also producible, inwhich the electrodes 22 a, 22 b, 22 c are located in the bottom portionof the membrane 9, on which electrodes rest the dielectric layer 21 thenthe bolometric layer 20.

The invention claimed is:
 1. A device for detecting electromagneticradiation, comprising: a substrate; at least one thermal detector placedon the substrate; and an encapsulating stricture encapsulating thethermal detector, including a thin encapsulating layer extending aroundand above the detector so as to define with the substrate a cavity inwhich the thermal detector is disposed, said thin encapsulating layerbeing produced from a material that is transparent to saidelectromagnetic radiation, wherein the thin encapsulating layercomprises a peripheral wall that encircles the thermal detector, andthat has a cross section, in a plane parallel to a plane of thesubstrate, of square or rectangular shape, corners of which are rounded.2. The detecting device according to claim 1, wherein the peripheralwall includes a rounded section and two rectilinear sections extendingalong, axes that are substantially orthogonal to each other, saidrectilinear sections being connected to each other by the roundedsection, the rounded section having a radius of curvature larger than orequal to two times a thickness of one of the rectilinear sections. 3.The detecting device according to claim 2, wherein the radius ofcurvature of the rounded section is measured from an internal surface ofthe rounded section that is oriented towards the cavity.
 4. Thedetecting device according to claim 1, wherein the thin encapsulatinglayer includes at least one through-orifice that is an exhaust vent, andwherein the encapsulating structure further includes a sealing layercovering the thin encapsulating layer so as to make the cavity hermetic.5. The detecting device according to claim 1, further comprising amatrix of detectors disposed in the cavity, in which the thinencapsulating layer further includes at least one section that is aninternal bearing section, disposed between two adjacent detectors, andthat bears directly against the substrate.
 6. The detecting deviceaccording to claim 5, wherein the internal bearing section has a profileof oblong shape in a plane parallel to a plane of the substrate, withrounded longitudinal ends.
 7. The detecting device according to claim 6,wherein the internal bearing section includes a sidewall portion and abottom portion, the sidewall portion extending substantially verticallyover an entire height of the cavity and the bottom portion contactingthe substrate.
 8. The detecting device according to claim 1, furthercomprising a matrix of thermal detectors, each detector of said matrixincluding a membrane configured to absorb the radiation to be detected,the membrane being suspended above the substrate and then sallyinsulated therefrom by anchoring pins and thermally insulating arms, thethin encapsulating layer including a plurality of through-orifices thatare exhaust vents disposed so that at least some of said thermaldetectors each have a single exhaust vent disposed facing acorresponding absorbing membrane and being plumb with a center of saidmembrane.
 9. The detecting device according to claim 8, wherein saidmembrane faces an exhaust vent and includes a through-orifice disposedplumb with said exhaust vent and being of larger or equal size to thatof said exhaust vent.
 10. The detecting device according to claim 9,Wherein said membrane includes a stack of a bolometric layer, adielectric layer that is structured so as to form two separate sections,and an electrically conductive layer that is structured so as to formthree electrodes, two of said electrodes, which are configured to beraised to a same electrical potential, flanking a third of saidelectrodes being a central electrode and which is configured to beraised to a different electrical potential, each electrode of said threeelectrodes contacting the bolometric layer, the central electrode beingelectrically insulated from the other two of said electrodes by thedielectric layer, the through-orifice passing through the centralelectrode and the bolometric layer in a zone disposed between the twoseparate Sections of the dielectric layer.
 11. The detecting deviceaccording to claim 10, wherein the encapsulating structure furtherincludes a sealing layer covering the thin encapsulating layer so as tomake the cavity hermetic, and wherein the substrate comprises a tielayer disposed facing the through-orifice of a corresponding membraneand configured to ensure adhesion of a material of the sealing layer.12. The detecting device according to claim 11, wherein the tie layerextends under a whole of the corresponding membrane and is made of amaterial configured to reflect the electromagnetic radiation to bedetected.
 13. A process for producing a detecting device for detectingelectromagnetic radiation, the detecting device comprising: a substrate,at least one thermal detector placed on the substrate, and anencapsulating structure encapsulating the thermal detector, including athin encapsulating layer extending around and above the detector so asto define with the substrate a cavity in which the thermal detector isdisposed, said thin encapsulating layer being produced from a materialthat is transparent to said electromagnetic radiation, wherein the thinencapsulating layer comprises a peripheral wall that encircles thethermal detector, and that has a cross section, in a plane parallel to aplane of the substrate, of square or rectangular shape, corners of whichare rounded; and the process comprising: placing the thin encapsulatinglayer on a sacrificial layer, and then removing the sacrificial layerand extracting the sacrificial layer through at least one exhaust ventin the thin encapsulating layer.